1. Field of the Invention
The present invention relates to an image forming apparatus and an image density detecting method.
2. Description of the Related Art
Typically, in an electrophotographic image forming apparatus, pseudo density gradation is expressed in binary data by implementing dithering.
While implementing dithering during image formation, in order to obtain the most suitable density gradation, a technology for adjusting the image density is known in which a plurality of differing dither patterns are formed and the image densities of those dither pattern images are detected by using an image density detection sensor. Then, the detection result is sent as a feedback to an image forming unit. According to the detection result, the image forming unit is controlled to perform image formation with the settings of that dither pattern which enables achieving the desired image density.
The abovementioned conventional technology is also implemented in an image forming apparatus in which a light-emitting diode (LED) array head including a plurality of LED arrays is used for the purpose of image formation. Usually, in the LED head array used for image formation, a plurality of LED arrays is aligned in the main-scanning direction. Each LED array includes a plurality of light-emitting elements. The LED arrays that are aligned in the LED array head form images on an image carrier, which performs relative movement in the sub-scanning direction on a line-by-line basis. While performing such image formation, problems may occur in the LED arrays that are arranged linearly and orthogonal to the sub-scanning direction of the image carrier. That is, misalignment may occur in the arrangement of the LED arrays thereby causing unevenness in that arrangement or misalignment may occur in the assembly of the LED arrays with respect to the LED array head. Such misalignment appears in the form of disturbance in the dither patterns, which are expected to be formed with regularity.
Hence, while adjusting the image density in the abovementioned manner, depending on the positioning of an image density detection sensor that detects the image density, the position at which disturbance occurs in a dither pattern may get detected by that sensor. Consequently, a noise image makes it difficult to properly detect the actual image density of the dither pattern.
Meanwhile, regarding dithering, Japanese Patent Application Laid-open No. 2010-061069 discloses a method by which the changes occurring in the image density due to skew correction can be corrected.
With the aim of preventing changes in the image density that occur in dither pattern images due to skew correction and with the aim of preventing the generation of stripe-shaped noise images that are formed periodically in the sub-scanning direction due to skew correction, Japanese Patent Application Laid-open No. 2010-061069 discloses a method in which correction is done based on image densities detected using pre-skew-correction dither patterns and post-skew-correction dither patterns and in which fine adjustment of image densities is done on a pixel-by-pixel basis.
However, the method disclosed in Japanese Patent Application Laid-open No. 2010-061069 is not intended for an image forming apparatus that includes light-emitting element arrays such as LED arrays. That is, in the light of the effects of stripe-shaped noise images formed due to misalignment of the light-emitting elements that occurs at the joining portions (joints) between the light-emitting element arrays, there is no way to properly detect the image densities of dither patterns. Thus, this issue remains unresolved.
There are problems occurring in an image forming apparatus that includes light-emitting element arrays such as LED arrays. Hence, there is a need to avoid the effects of stripe-shaped noise images formed due to misalignment of light-emitting elements such as LED arrays that occurs at the joining portions (joints) between the light-emitting element arrays, and to properly detect the image densities of dither patterns.